Park and walk. That sounds flippant, but it is serious. A car is very inefficient when driven at low speeds through a parking lot, so take the first parking spot you see, even if it means a long walk to the store. You can greatly improve efficiency if you wait until you need several things, so you can get them all in one trip. Or simply walk to a nearby store instead of driving there.
If your car has a mile per gallon computer, you can learn how to drive to get improved numbers. For instance, when you approach a traffic signal, you can slow down so you don't have to stop. Every time you hit the brake you waste the gas it takes to get rolling again. If the car is going downhill, you can take your foot off the gas pedal and let it coast. The MPG will usually go over 100 when you do that. I find that driving by the MPG reading raises the average by about two points.